The invention relates to a feedstock recycling process for thermoset materials.
An issue which is increasingly raised in the industrial use of plastics, alongside low-cost availability, good processing performance, and advantageous physical properties, is the destiny of the plastics after use. When considering desirable material usage cycles, the principle is: reuse is preferable to other recycling methods, which are preferable to disposal.
Since in many sectors of industry the reuse of plastics is severely restricted, the spotlight is being placed on the requirement for methods of recycling. In this connection, since they can be melted by heating, thermoplastics are in principle suitable for product recycling. However, materials which have been filled or reinforced, for example with glass fibers, give problems. In contrast, this procedure is impossible with thermosets, i.e. cured plastics which cannot be melted. But it is just these thermoset materials which are the materials of choice for many products in electrical engineering and electronics, since the materials have excellent mechanical and thermal properties. This is particularly true for fiber-reinforced materials and plastic-based composites.
In Germany some hundreds of thousands of metric tons of glass-fiber-reinforced plastics (GRPs) are processed every year. More than 50% of these GRP materials are thermoset materials. The resin systems used comprise amino resins, epoxy resins, phenolic resins, and unsaturated polyester resins. Epoxy resins have proven particularly successful for high-performance composite materials. Besides glass-fiber-reinforced epoxy resin materials, the materials used here include those reinforced with carbon fibers or with aramid fibers.
Since increased awareness of the environment means that in the future there will be regulations in almost every case requiring the manufacturer to take back used products, the producers of fiber-reinforced thermoset materials are increasingly also having to provide ways of recycling these materials.
Energetic recycling (incineration with regeneration of energy) has only limited advantages for fiber-reinforced or highly filled plastics, since the inert filler content and fiber content is up to 70% and has to be disposed of as waste, mostly as hazardous waste. In addition, incineration of halogen-containing plastics produces hydrogen halides, which require complicated removal by washing, and require neutralization.
Product recycling of thermoset materials is currently possible only in the form of particle recycling. For this, the materials have to be comminuted, ground, treated and fractionated, and, where appropriate, purified. In the case of printed circuit boards, the metallic particles have to be separated off from the fiber/plastic content. Proportions of up to 15% of the recycled plastic can then be mixed as a filler with thermoplastics or thermosets. Since the amount is limited to 15%, however, there is little opportunity to recycle returned parts; in most cases the process is therefore limited to the reincorporation of waste arising in situ during processing. Since the material cannot be recycled to a sufficient extent, there is clearly a need for feedstock recycling to be advanced beyond the initiatives seen hitherto.
Base chemicals in petrochemistry can be produced by degrading polymer structures, thus saving resources. The products obtained can thenxe2x80x94directly or after treatmentxe2x80x94be utilized as raw materials for plastics production. The available modes of feedstock recycling are as follows:
pyrolysis
gasification
hydrolysis and oxidation using sub- or supercritical water
hydrogenation
Pyrolysis is thermal decomposition of organic compounds without a supply of oxygen, forming gases, condensable products, and solid, carbon-containing residues. It is carried out at temperatures of from 550 to 1100xc2x0 C. The products obtained are pyrolysis gas, pyrolysis oil and pyrolysis coke, and the product specifications accessible here depend on the pyrolysis temperature range. Various pyrolysis plants using plastics, which have been in operation only for a short time, have shown that the pyrolysis oils which can be used for product recycling are rich in oxygen and thermally unstable, so that the process of treatment to give base materials for petrochemistry is relatively complicated. The yield of pyrolysis products which can be reused directly in the process of plastics production is presently about 33%.
Gasification involves partial oxidation. Here, plastic wastes are reacted with oxygen and, where appropriate, steam, as gasifier, at temperatures of from about 1300 to 1500xc2x0 C. to give a mixture of carbon monoxide and hydrogen (synthesis gas), carbon dioxide, and also methane, ethylene, and acetylene. A solid residue of about 2% of carbon black with ash is left behind. The heat of reaction liberated means that the gasification process can be carried out autothermally. The synthesis gas, which still has about 75% of the energy content of the starting materials, can be used to prepare methanol and products derived from methanol.
The hydrolysis and oxidation of plastics using sub- or supercritical water takes place in a closed vessel at temperatures up to 500xc2x0 C. and pressures up to 1000 bar. The supercritical point is at 374xc2x0 C. At this temperature the pressure reached is 221 bar. Without oxidant it is solely hydrolysis which takes place. Controlled attack of water molecules at the linkage points in polycondensates causes them to cleave, enabling the starting materials to be reclaimed. Oxidants which may be used are air, oxygen, and hydrogen peroxide. These oxidize the organic skeleton of the plastic to give carbon dioxide and water. The nitrogen content in nitrogen compounds is converted into ammonia or nitrogen.
In hydrogenation, plastics wastes give fuels, heating oils, heating gases or petrochemical raw materials via addition reactions with hydrogen at temperatures up to 480xc2x0 C. and pressures up to 300 bar. The yield achieved here of hydrogenated oils which can be treated in refineries of the usual type is about 80%. However, this yield can only be achieved if the plastics are processed together with oil residues (plastics content from 8 to 10%), and are used in a dechlorinated and dried condition.
The use of 1,2,3,4-tetrahydronapthalene (tetralin) as a hydrogen donor has been proposed as an alternative to pressure-hydrogenation with hydrogen for degrading crosslinked, i.e. fully cured, phenol-formaldehyde resins (see: D. Braun, R. Steffan, Angew. Makromol. Chem. 92 (1980), pages 169 to 189). This procedure corresponds to what is known as the Pott Broche process for extractive coal liquefaction (see: A. Pott, H. Broche, Fuel 13 (1934), pages 91 to 95, 125 to 128 and 154 to 157). This process can give a conversion of more than 90% in the liquefaction of coal with tetralin.
It has been found that when phenol-formaldehyde resins are cleaved by hydrogenolysis, these resinsxe2x80x94in particle form (grain sizexe2x89xa62 mm)xe2x80x94at temperatures of 410xc2x0 C., using tetralin, are degraded within a period of 5 h predominantly to give low-molecular-weight, tetralin-soluble phenolic cleavage products. The pressures reached here are up to 26 bar. After cooling the pressure is only slightly superatmospheric, due to small proportions of gaseous cleavage products. Depending on the reaction time and reaction temperature, considerable amounts of insoluble, carbonized degradation products are obtained.
It is an object of the invention to provide a process which permits the feedstock recycling of thermoset materials, in particular of epoxy resin materials, under the mildest possible conditions with low energy cost and high yield of recycled base chemicals. A particular object here is to find a method which when used on composite thermoset materials even permits the components in the composite to be reclaimed in the purest possible form.
According to the invention, this is achieved by treating a thermoset material with a partially hydrogenated aromatic hydrocarbonxe2x80x94in the presence of an aminexe2x80x94at a temperature of from 200 to 350xc2x0 C.
Specifically and surprisingly, it has been found that the hydrogenating, i.e. the hydrogenolytic cleavage, of thermoset materials, in particular composite thermoset materials, using a partially hydrogenated aromatic hydrocarbonxe2x80x94in the presence of a compound having an aminic structural unitxe2x80x94can give gentle but quantitative cleavage into low-molecular-weight compounds, even at temperatures as low asxe2x89xa7200xc2x0 C. This process firstly gives a saving in energy (lower costs) and secondly avoids formation of any tar- or pitch-like residues during the hydrogenolysis, since the conditions are mild. This means not only that the organic cleavage products are obtained at high concentrations, but also that embedded components and the components in the composite can be reclaimed quantitatively in pure form.
Thermosets are crosslinked plastics produced from curable resins. The invention is particularly applicable to thermoset materials based on the following industrially important groups of materials: amino resins, epoxy resins, phenolic resins, and unsaturated polyester resins. Epoxy resin materials are particularly advantageously subjected to this recycling process.
Examples of organic cleavage products reclaimed, almost quantitatively, during the recycling process are phenols when phenolic resins are used and phthalic acids when anhydride-cured epoxy resins are used. When epoxy resins cured with amines are used, high concentrations of amines are produced. Other materials obtained are benzene, phenol, cresols, and isopropanol, and also other monomers.
Another advantage of the invention is that the materials do not have to be in particle form, i.e. do not have to be comminuted prior to cleavage by hydrogenation, but can be used in the form of an entire component. This is particularly important for components which derive from electrical engineering or electronics and comprise a wide variety of different materials in combination with thermoset molded materials, in particular with epoxy resin molded materials. This method enables, for example, entire printed circuit board assemblies, casings, or modules of relatively large components to be introduced into the recycling process without complicated pretreatment. Another point of very great importance is that when carbon-fiber-reinforced thermoset materials are used the carbon fibers are reclaimed in pure form. It is very surprising, since the Pott-Broche process also hydrogenates carbon.
Particular partially hydrogenated aromatic hydrocarbons used, i.e. hydrogen donors, are 1,2,3,4-tetrahydronaphthalene (tetralin) and 9,10-dihydroanthracene. Partially hydrogenated aromatics which may be used as hydrogen donors in the cleavage by hydrogenation are generally those which have a high tendency to rearomatize. Examples of such compounds are fluorene, phenylene, xanthene, 7H-benzo[d,e]anthracene and indoline (2,3-dihydro-1H-indole). It is also advantageous to use mixtures of partially hydrogenated aromatic hydrocarbons. An example of a mixture of this type is one composed of indoline and 1,2,3,4-tetrahydronaphthalene.
In the process of the invention, the hydrogenolytic cleavage of the thermoset materials takes place in the presence of an amine or, respectively, of a compound having an aminic structural unit. It is possible to use an amine directly here. However, if the aromatic hydrocarbon itself contains an appropriate structural unit, as is in particular the case with indoline, then the addition of the additional amine can be dispensed with. It is also possible to dispense with any separate addition of an amine in cases where the thermoset material has an aminic structural unit and release of an amine is therefore possible during the cleavage by hydrogenation, as is in particular the case when using amine-cured epoxy resin molded materials.
The amines used may be aliphatic or aromatic mono-, di- or oligoamines, and these may be primary or secondary amines. Preferred amines used are ethanolamine and octylamine. The following compounds may also be used: nonylamine, decylamine, undecylamine, dodecylamine, cyclohexylamine, ethylenediamine, 1,3-diaminopropane, 1,3-diamino-2-propanol, 1,4-diaminobutane, 1,5-diaminopentane, 1,6-diaminohexane, 1,7-diaminoheptane, 1,8-diaminooctane, 1,9-diaminononane, 1,10-diaminodecane, 1,12-diaminododecane, 1,2-diaminocyclohexane, diethylenediamine, polyethyleneimine, tris-(2-aminoethyl)amine, bis-(3-aminopropyl)amine, O,Oxe2x80x2-bis-(2-aminoethyl)-ethylene glycol, O,Oxe2x80x2-bis-(2-aminoethyl)polyethylene glycol, piperazine, indoline, piperidine, benzylamine, aniline, diaminotoluene, diaminopyridine, 4,4xe2x80x2-diaminodicyclohexylmethane and 4,4xe2x80x2-diaminodiphenyl ether.
Surprisingly, it has also been found that epoxy resin materials themselves then undergo hydrogenolytic cleavage even if no amine or compound having an aminic structural unit is present. The invention therefore also provides a process for the feedstock recycling of thermoset materials based on epoxy resins, by treating an epoxy resin material with a partially hydrogenated aromatic hydrocarbon at a temperature of from 300 to 350xc2x0 C. This therefore gives a way of recycling in particular anhydride-cured epoxy resin molded materials and, respectively, epoxy resin materials and composite epoxy resin materials, including those which comprise carbon fibers.
The extraordinary significance of the process of the invention lies in the fact that the thermoset materials are hydrogenated so gently that firstly feedstock recycling of the plastic components takes place and secondly that fillers or embedded components or components in the composite can be reclaimed and reused quantitatively and in pure form. In electronics and electrical engineering, this method also enables the metals used in electronic circuits to be regained in pure form. Another advantage of this process is that halogen-containing thermoset materials can also be used, but without hydrogen halides.
The cleavage by hydrogenation may take place at atmospheric pressure under reflux, or under pressure in a pressure vessel, depending on the hydrogen donor used and its boiling point. An example of a pressure vessel which may be used is an autoclave or a sealed tube. In the course of the cleavage of the thermoset material by hydrogenation a certain pressure builds upxe2x80x94temporarilyxe2x80x94in the pressure vessel.
Materials which may be used in the process of the invention are cured thermoset molded materials per se and also filled molded materials. However, the use of composite materials is preferred, examples of the reclaimed materials then being glass fibers or glass-fiber fabrics. This process is moreover particularly important for composite thermoset materials, especially composite epoxy resin materials, which comprise carbon fibers, or carbon-fiber fabrics, or metals, such as gold or copper, or comprise components. In the feedstock recycling of materials of this type the components in the composite are regained in pure form.